Process for treating hydrocarbon streams

ABSTRACT

A process is disclosed for treating a hydrocarbon stream including flowing the hydrocarbon stream through a hydrocarbon treating vessel, heating a portion of the hydrocarbon treating vessel to a predetermined temperature and for a predetermined amount of time and controlling sensitization of the portion of the interior surface of the hydrocarbon treating vessel.

STATEMENT OF PRIORITY

This application is a national stage entry under 35 U.S.C. 371 ofInternational Application No. PCT/US2011/054163 filed Sep. 30, 2011,which claims benefit of U.S. Provisional Application Nos. 61/469,948,61/469,944, and 61/469,932, which were all filed on Mar. 31, 2011.

FIELD OF THE INVENTION

The field of the invention is treating hydrocarbon streams.

BACKGROUND OF THE INVENTION

Oil refineries typically incorporate one or more different processes fortreating and/or converting hydrocarbons, such as, for example, thosepresent in crude oil or other naturally occurring source, to producespecific hydrocarbon products with properties that are useful forparticular applications.

To carry out the hydroprocessing operations to treat crude oil and otherhydrocarbons to form usable products, oil refineries typically includeone or more complexes or groups of equipment designed for carrying outone or more particular treating or conversion processes to preparedesired final products. In this regard, the complexes each may have avariety of interconnected units or vessels including, among others,tanks, furnaces, distillation towers, reactors, heat exchangers, pumps,pipes, fittings, and valves.

Many types of hydrocarbon treating operations are carried out underrelatively harsh operating conditions, including high temperaturesand/or pressures and within various harsh chemical environments. Inaddition, due to the large demands for hydrocarbon and petrochemicalproducts, the volumetric flow rate of a hydrocarbon stream throughvarious oil refinery complexes is substantial, and the amount ofdowntime of the processing equipment is preferably small to avoid lossesin output.

High temperature hydrocarbon treating operations generally involveheating a hydrocarbon stream to a process temperature and flowing thehydrocarbon stream through one or more hydrocarbon treating vesselsforming a refinery complex. Specific process techniques are utilizeddepending on the feed and the desired products, and may include flowingthe hydrocarbon stream in the presence of other materials and/orreactants, including gases and liquids, adsorbents to remove particularcomponents from the product stream, and/or catalysts to control reactionrates. In this manner, the hydrocarbon stream can be treated to, forexample, modify one or more components within the hydrocarbon stream,react one or more components with other materials (e.g. gases) within avessel, and remove components from the hydrocarbon stream either aspotential products, sometimes upon further processing, or for disposal.

Traditionally, austenitic stainless steels have been used to fabricatethe oil refinery vessels listed above, because these types of alloys areuseful in a variety of harsh environments. The addition of 8% nickel toa stainless steel containing 18% chromium produces a remarkable changein microstructure and properties. The alloy solidifies and cools to forma face-centered cubic structure called austenite, which is non-magnetic.Austenitic stainless steels are highly ductile, even at cryogenictemperatures and have excellent weldability and other fabricationproperties.

Many metals, including austenitic stainless steels, can be subject to ahighly localized form of corrosion known as stress-corrosion cracking(SCC). SCC often takes the form of branching cracks in apparentlyductile material and can occur with little or no advance warning. In lowpressure vessels, the first sign of stress corrosion cracking is usuallya leak, but there have been instances of catastrophic failures of highpressure vessels due to stress corrosion cracking. Stress corrosioncracking occurs when the surface of the material exposed to a corrodingmedium is under tensile stress and the corroding medium specificallycauses stress corrosion cracking of the metal. Tensile stresses may bethe result of applied loads, internal pressure in piping systems andpressure vessels or residual stresses from prior welding or bending.

Austenitic stainless steels can be subject to stress corrosion crackingin, for example, hot chloride solutions, hot caustic soda and hotsulfides or polythionates. Specifically, polythionic acid stresscorrosion cracking has been found to occur within refinery complexvessels due to the presence of even small quantities of sulfur contentthat is either added during a refinery process or is present in thefeedstock. The risk of polythionic acid stress corrosion crackinggenerally increases in temperature ranges of between 370 and 815° C.

In order for polythionic acid stress corrosion cracking to occur inaustenitic stainless steels, typically the steel must first undergosensitization and either concurrently or subsequently be subjected to acorrosive agent, such as polythionic acid. For example, unstabilizedgrades of austenitic stainless steels such as types 304 and 316,traditionally used in the fabrication of oil refinery complexes, haveall exhibited sensitization and polythionic acid stress corrosioncracking due to polythionic acid. Even the stabilized grades such astype 321 and 347 can exhibit sensitization and polythionic acid SCC.Typically, chromium within the austenitic stainless steels reacts withoxygen to form a chromium barrier that protects the material fromcorrosion. At high temperatures, however, usually somewhere at or 370and 815° C. depending on the stainless steel alloy, chromium-richcarbides precipitate out at the grain boundaries. The precipitation ofchromium depletes the chromium content adjacent to the grain boundariesforming chromium depleted zones and drastically reducing the corrosionand/or cracking resistance in corrosive environments in these zones.PTA-SCC requires the combination of sulfide scale formation on the metalsurface, sensitized microstructure, tensile stress, moisture and oxygen.

FIG. 1 from D. V. Beggs and R. W. Howe, “Effects of welding and ThermalStabilization on the Sensitization and Polythionic Acid Stress corrosionCracking of Heat and Corrosion-Resistant Alloys”, NACE Conference 1993,Paper no. 541, illustrates the temperatures and times at whichtraditional austenitic stainless steels have been found to exhibitsensitization. As can be seen from the figure, the peak temperatures andtimes for sensitization of austenitic stainless steels are materialspecific, although they generally occur within a temperature range ofbetween 565° and 650° C. Specifically, type 347 stainless steel exhibitspeak sensitization at 565° C., (i.e. exhibits sensitization at thistemperature faster than at higher or lower temperatures) but does notsensitize at this temperature until after 1,000 hours of being held atthe elevated temperature. Type 347 stainless steel is often used inrefinery processing equipment due to the longer time that it canwithstand sensitization when compared with other stainless steels asshown in FIG. 1. As illustrated in FIG. 1, each stainless steel alloyexhibits a different sensitization envelope, i.e., area on atime/temperature diagram where the alloy exhibits sensitization.

One type of harsh corrosive environment to which sensitized stainlesssteels are particularly susceptible is one that contains polythionicacid (PTA) formed from the decomposition of sulfide scale by moisture inair. Due to the high temperature of operation and the presence of sulfur(S) and hydrogen sulfide (H₂S) in a reducing environment or in a feedstream in many oil refinery complexes and/or processes, an iron sulfidescale can form on stainless steel surfaces. Upon shutdown of theequipment, if the sensitized stainless steel is exposed to moisture andoxygen from the surrounding environment, there is the potential that themetal can crack as a result of polythionic acid stress corrosioncracking (PTA-SCC). In other words, the sulfur and hydrogen sulfide willreact with oxygen and moisture from the ambient environment to formpolythionic acid. Due to the existence of the chromium depleted zonesformed by sensitization, the PTA can attack these zones causingcorrosion and ultimately PTA-SCC where the vessel is put under tensilestresses either by being pressurized or by having residual stressesfrom, for example, welding during fabrication.

Commercially, internal surfaces of refinery complex equipment forcarrying out processes at elevated temperatures are usually made of Type304 and Type 347 austenitic stainless steels, especially for use insulfur or H₂S-containing reducing environments, such as for examplehydroprocessing and hydrocracking reactors, heaters and heat exchangers,complexes for converting of liquid petroleum gas (LPG) into aromaticsthrough dehydrocyclodimerization, and processes for catalyticdehydrogenation for the production of light olefins from paraffins. Themost widely used stainless steel is probably Type 304, sometimes calledT304 or simply 304, because of cost. Type 304 stainless steel is anaustenitic steel containing 18 to 20% chromium and 8 to 10% nickel. Thisand other specialty austenitic stainless steels have been used in theseapplications due to the high temperature H₂S and sulfur corrosion andhigh temperature hydrogen attack issues that are present in theseprocesses.

In some instances, protective coatings are applied to protect theoutside of stainless steel vessels from exposure to chlorides ininsulating jackets. In other applications, post welding heat treatmentcan be used to relieve residual stress in the steel alloys. The risk ofPTA-SCC in oil refinery equipment has heretofore primarily beenaddressed by known processes to either prevent the formation of PTA orto neutralize the PTA in the environment prior to exposure to air.

Preventing PTA formation can be accomplished by either eliminatingliquid phase water or oxygen, since these are the components responsiblefor reacting with the sulfide scale to form the PTA. One approach is tomaintain the temperature of the austenitic stainless steel equipmentabove the dew point of water to avoid condensation of the moisture.Another approach is to purge the equipment with a dry nitrogen purgeduring any shutdown or startup procedure, when the system isdepressurized and the equipment is opened and exposed to air, since thisis generally the only time when significant amounts of oxygen mightenter the system. The dry nitrogen purge restricts ambient oxygen andmoisture from entering the system.

On the other hand, PTA that has or is likely to form within a complex orvessel may be neutralized by an ammoniated nitrogen purge or an aqueoussolution of soda ash. In the case of utilizing an ammoniated nitrogenpurge, special procedures are utilized to form the ammoniated nitrogen,which is pressurized and blown into the system. On the other hand, asoda ash solution neutralization step involves completely filling thepiping or piece of equipment involved with the solution and allowing theequipment to soak for a minimum of two hours prior to exposing thesystem to air. Each of these processes is time consuming and impracticalduring the operation of an oil refinery complex as it requiresadditional materials and additional downtime of the particular equipmentto perform the purge or neutralization steps. In addition, due to thepresence of the nitrogen, ammoniated nitrogen, or soda ash, specialprecautions must be taken to protect service workers working on theequipment when these materials are present. Also the removal of thesechemicals reduces the need for special handling and waste disposal. Iftrace levels of the chemicals remain, which is often the case, catalystin the reactor can be poisoned.

In addition, chemically stabilized austenitic stainless steels likeTP321 and TP347 have been used in reactors that processsulfur-containing streams because of their resistance tohigh-temperature corrosion through desulfurization. However, suchaustenitic stainless steels are also susceptible to stress corrosioncracking as a result of exposure to polythionic acid, since it is just amatter of time and temperature for them to sensitize, which falls withinthe operating conditions of many hydrocarbon treatment processes.Although TP321 and TP347 are generally used in applications according tothe above methodologies in petroleum refinery industries, the need forpost-weld heat treatment and for special procedures during shutdown andstartup of a refinery complex affect not only costs but also productiontime since they take a certain amount of time to carry out.

There is a continuing need, therefore, for improved processes fortreating hydrocarbon streams while avoiding expensive, time consumingand inconvenient additional steps for purging or neutralizing theinternal environment to avoid forming polythionic acid withinhydrocarbon treating vessels and causing PTA-SCC.

BRIEF SUMMARY OF THE INVENTION

According to one approach, a process is provided for treating ahydrocarbon stream. The process includes flowing the hydrocarbon streamthrough a hydrocarbon treating vessel. In addition, the process includesheating at least a portion of an interior surface of the vessel to apredetermined vessel temperature of 565° C. or greater for 1,000 hoursor more. In this regard, the vessel is heated at a temperature and for atime where sensitization of the portion of the interior surface wouldnormally occur. The process further includes controlling thesensitization that occurs in the portion of the hydrocarbon treatingvessel by employing a hydrocarbon treating vessel with at least theportion thereof formed of a low-carbon stainless steel alloy having0.005 to 0.020 wt-% carbon, 9.0 to 13.0 wt-% nickel, 17.0 to 19.0 wt-%chromium, 0.20 to 0.50 wt-% niobium, and 0.06 to 0.10 wt-% nitrogen torestrict sensitization of the portion of the interior surface.Surprisingly, it has been found that sensitization of the portion of theinterior surface of the hydrocarbon treating vessel is reduced orrestricted, even though the portion is heated to a temperature and for atime that would typically result in sensitization for hydrocarbontreating vessels formed of traditional austenitic stainless steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing sensitization envelopes of traditionalaustenitic stainless steel alloys.

DEFINITIONS

The term “vessel” means any type of container, tank, reactor, pipe,tower, column, exchanger or other structure or apparatus within an oilrefinery complex that holds a hydrocarbon fluid or allows a hydrocarbonstream to flow therethrough on a continuous or a batch or intermittentbasis during operation of the oil refinery complex.

The term “hydrocarbon treating vessel” means a vessel within an oilrefinery complex

The term “maintaining” means that the material flow is maintained for anindicated period of time, but may be interrupted for maintenance orservice. As used herein, a hydrocarbon flow is maintained even though itmay be interrupted for routine or unexpected maintenance, service, orrepair.

The term “interior surface” means any exposed surface within ahydrocarbon treating vessel, including both the vessel interior walls aswell as any other structures within the vessels such as screens, tubes,internal equipment etc.

DETAILED DESCRIPTION

A process for treating a hydrocarbon feed stream including one or moredifferent hydrocarbons and which may include other components and/orimpurities is provided that includes flowing the hydrocarbon streamthrough a hydrocarbon treating vessel. The hydrocarbon treating vesselmay be included as part of a larger oil refinery complex capable ofperforming one or more particular types of hydrocarbon conversion ortreatment processes for converting or treating one or more components ofthe hydrocarbon feed stream to form a desired product. The processincludes flowing the hydrocarbon stream into the hydrocarbon treatingvessel for treatment thereof. Heat is applied to the hydrocarbon streamand/or the vessel during operation. Heat may be applied to thehydrocarbon stream while it is within or before entering the hydrocarbontreating vessel to raise the temperature thereof to a processtemperature. In this manner, the hydrocarbon treating vessel is alsoheated to a predetermined vessel temperature either by heating thehydrocarbon stream therein or by heat transfer from the hydrocarbonstream to the vessel walls. Particular process parameters or operatingconditions, such as temperature, pressure, and space velocity, aretypically process specific and are selected to promote the particularreactions or treatment steps of the particular process.

In one approach the process is maintained for a predetermined amount oftime. In this regard, it should be noted that the process may be shutdown intermittently for servicing or replacement of equipment,inspection, or for other reasons. In other words, other than periodicand/or intermittent shutdowns, according to this approach, the processis maintained for the predetermined amount of time, including flowingthe hydrocarbon stream through the hydrocarbon treating vessel andheating the hydrocarbon stream and the vessel so that the vessel ismaintained at the predetermined temperature.

In one approach, the process includes controlling sensitization of aportion of the interior surface of the vessel that is heated to apredetermined vessel temperature. Controlling sensitization includesrestricting or reducing the amount of sensitization that occurs and mayinvolve restricting or reducing the extent of precipitation of chromiumcarbides within the material of the portion of the interior surface ofthe hydrocarbon treating vessel. The precipitation of chromium carbidesis controlled even though the hydrocarbon treating vessel is heated to apredetermined vessel temperature for a predetermined amount of time, toa point where sensitization is typically observed within a sensitizationenvelope of a traditional austenitic stainless steel. Controllingprecipitation of chromium carbides within the interior surface of thehydrocarbon treating vessel may be achieved by heating at least aportion of the interior surface of the hydrocarbon treating vesselformed from a sensitization resistant stainless steel.

In one approach, the interior surface of the hydrocarbon treating vesselmay be heated above a predetermined vessel temperature by the flow ofthe hydrocarbon stream therethough, where the hydrocarbon stream isheated to the predetermined temperature before entering the vessel, andheat is transferred from the hydrocarbon stream to the interior surface.In another approach, the interior surface of the hydrocarbon treatingvessel may be heated above the predetermined vessel temperature byapplying heat to the vessel or internal equipment or structure by use offired heaters, heat exchangers, or other heating equipment in order toraise the temperature of the hydrocarbon stream flowing therethrough toprocess temperatures.

The hydrocarbon treating vessel may be heated to a predetermined vesseltemperature and maintained at the predetermined vessel temperature forthe predetermined amount of time. It has been found that by heating thehydrocarbon treating vessel formed of the low-carbon stainless steel toa predetermined time and temperature that falls within normal operatingconditions of high temperature hydrocarbon treating processes,sensitization of the hydrocarbon treating vessel does not occur.Surprisingly, sensitization has been reduced or restricted even wherethe predetermined vessel temperature and the predetermined time formaintaining the process fall within or near the sensitization envelopeof austenitic stainless steels traditionally used for the fabrication ofhydrocarbon treating vessels. Not to be bound by theory, it is believedthat the lower carbon content in the new low-carbon stainless steelreduces or restricts the extent of precipitation of chromium carbideswithin the alloy along the grain boundaries. This in turn reduces orrestricts the formation of chromium depleted zones and the resultingsensitization that typically is present in austenitic stainless steelsused for oil refinery complex fabrication. It is further believed thatthe addition of niobium interacts with the carbon and nitrogen that ispresent in the material to restrict the formation and precipitation ofchromium carbides. It is also believed that the addition of nitrogen inthe low-carbon stainless steel reduces any loss in strength of thehydrocarbon treating vessel that may otherwise occur due to the lowcarbon content.

Turning to more of the particulars, an oil refinery complex for carryingout one or more particular hydrocarbon conversion or treating processes,and particular hydrocarbon treating vessels therein, in accordanceherewith, includes equipment for transporting and holding thehydrocarbon stream and for promoting the processes taking place in thecomplex and/or a vessel. The particular equipment within a given complexwill depend on, among other things, the feed and desired products, theprocesses being carried out, and the operating conditions, including theoperating temperatures, pressures, and space velocities.

The equipment may include one or more hydrocarbon treating vessels thatmay facilitate the flow of the hydrocarbon stream though the complex,contain the hydrocarbon stream therein, and/or promote particularprocess or processes being accomplished within the complex. Thehydrocarbon treating vessels may include, for example, tubing or pipingfor transporting the hydrocarbon stream and or streams of othermaterials, such as recycle streams, treatment gases, and catalysts. Thepiping is typically in the form of hollow pipes with pipe walls having awall thickness and interior surfaces thereof for directing the flow of ahydrocarbon stream or other material therethrough. Additional structuresor discontinuities, such as flanges for connecting pipes together andwelds for welding sections of pipe together may also be provided.Nozzles and/or valves may also be incorporated with the piping or othervessels within the complex for controlling the flow of the hydrocarbonstream or other materials through the complex.

Many hydrocarbon treating processes include particular equipment forraising the temperature of the hydrocarbon stream to predeterminedprocess temperatures. For example, hydrocarbon treating vessels within acomplex may include combined feed heat exchanger having tubing forrunning streams adjacent to one another to transfer heat therebetween.For example, the heat exchanger may include piping or other structurefor running a hot stream, such as effluent leaving a reactor, adjacentto a cooler stream, for example a feed stream entering a reactor, toincrease the temperature of the feed stream to process temperatures. Inaddition, the complex may include heating elements, such as heatingtubes with fired heaters for heating the hydrocarbon feed stream by heattransfer through the heating tubes to raise the temperature of thehydrocarbon feed stream to process temperatures.

An oil refinery complex will also typically include one or more reactorsfor carrying out one or more treatment steps. For example, a hydrocarbontreating vessel within a complex may include a reactor that facilitatesa particular chemical reaction for converting or treating at least onecomponent of the hydrocarbon stream. To this end, the reactor mayinclude a catalyst for promoting the chemical reaction and/or anothermaterial, such as a gas, that reacts with the hydrocarbon stream orpromotes chemical reactions of the hydrocarbon stream to treat orconvert a component of the hydrocarbon stream. The reactor may also beprovided to treat the hydrocarbon stream in other manners, such asselectively removing a particular component from the hydrocarbon stream.For example, the reactor may include an adsorbent that selectivelyadsorbs a particular component of the hydrocarbon stream to remove thecomponent from the hydrocarbon stream to be discarded or captured as aproduct for distribution or further processing.

Internal equipment and structures are also typically included within theequipment for various reasons. For example, reactor internals mayinclude screens, flanges, flow interrupters or directors, and otherstructures and equipment for facilitating particular hydrocarbontreating processes. The equipment may be provided for directing the flowof the hydrocarbon stream or other materials within the hydrocarbontreating vessels of the complex. For example, the equipment may beprovided for directing the hydrocarbon stream in a certain pattern inorder to maximize the exposure of the hydrocarbon stream to othermaterials, such as, for example a catalyst, adsorbent, and/or a reactionmaterial (e.g. hydrogen gas) within the vessel. The internal equipmentmay also be provided for holding or moving materials within the vessels,such as screens or packed bed materials that hold a catalyst and allowthe hydrocarbon stream to pass therethrough.

In addition, a particular complex for carrying out a specifichydrocarbon treating process may include additional equipment and/orprocess specific equipment or structures. It is contemplated that theterm hydrocarbon treating vessel as used herein includes any of theabove described equipment and structures and any other equipment orstructures within a particular oil refinery complex. It is alsocontemplated that internal surfaces of a hydrocarbon treating vessel asused herein includes inner surfaces of the hydrocarbon treating vesselwalls and any other structures thereof, as well as surfaces of internalequipment and structures that are exposed within the vessel.

According to one approach the process includes flowing a hydrocarbonstream through the hydrocarbon treating vessel. The hydrocarbon streammay continuously flow through the hydrocarbon treating vessel, or thehydrocarbon stream may flow through the hydrocarbon treating vessel onan intermittent or batch basis. Treatment of the hydrocarbon stream mayoccur within the hydrocarbon treating vessel, or it may occur in aseparate vessel within the hydrocarbon treating complex upstream ordownstream from the hydrocarbon treating vessel.

In one approach, the operating of the hydrocarbon treating complex, andthe particular hydrocarbon treating process is maintained for apredetermined period of time. Maintaining operation of the hydrocarbontreating vessel includes maintaining the flow of the hydrocarbon streamor other material therethrough and maintaining the temperature of thevessel at the predetermined temperature. As used herein, maintainingoperation of the hydrocarbon treating process, complex, or vesselincludes operation that is maintained for the predetermined period oftime, although the operation may be intermittently or periodicallyinterrupted or shut down for servicing or inspection of the equipment,as is typical in a hydrocarbon treating process. The predeterminedtemperature as used herein does not necessarily refer to a knowntemperature, and may include an approximate temperature or a temperaturefalling within a known range of temperatures.

In one approach, the process includes maintaining operation of thehydrocarbon treating vessel or complex for above 1,000 hours, withintermittent process shutdowns during the period of time, withoutsensitization of the interior surfaces of the vessel occurring. Inanother approach, the process is maintained for a predetermined amountof time of above 5,000 hours without sensitization of the interiorsurface occurring. In yet another approach, the predetermined period oftime is 10,000 hours or greater without sensitization of the interiorsurface of the vessel occurring. It should be noted that as describedherein, sensitization is not considered to occur where the amount ofsensitization of the internal surfaces within the hydrocarbon treatingvessel that occurs is insufficient to cause polythionic acid stresscorrosion cracking of the equipment within the predetermined amount oftime.

In one approach, the process includes a high temperature process forconverting a hydrocarbon stream. In this approach, the hydrocarbonstream is heated to a high process temperature during treatment thereof.In this regard, at least a portion of the interior surface of thehydrocarbon treating vessel is heated to a vessel temperature by, forexample, direct heating of the vessel in order to heat the hydrocarbonstream flowing therethrough or through heat transfer from an alreadyheated hydrocarbon stream flowing through the vessel to the vesselinternal surfaces. In one approach, at least a portion of the internalsurface of the vessel is heated to a predetermined vessel temperature.In another approach, some or all of the internal surfaces of the vesselor a plurality of hydrocarbon treating vessels within a hydrocarbontreating complex are heated to a predetermined temperature of above 400°C. In another approach, the predetermined vessel temperature is above550° C. In yet another approach, the predetermined vessel temperature is565° C. or greater. In still another approach, a maximum predeterminedvessel temperature is below 700° C. In this manner, during the process,the vessel is heated within a temperature range that is within typicaloperating parameters of hydrocarbon treating processes and complexes.The predetermined temperatures are also within a range at which the peaksensitization of similar austenitic stainless steels usually occurs, asshown by the sensitization envelopes in FIG. 1, while restricting orreducing sensitization thereof. As used herein, the term predeterminedtemperature does not necessarily refer to a constant or knowntemperature, and may include, for example, an average temperature, amedian temperature, a temperature range, and the like.

In one approach, the process includes controlling sensitization of atleast a portion of an interior surface of the hydrocarbon treatingvessel by employing a portion of the interior surface that is formedfrom a new low-carbon stainless steel. The low-carbon stainless steelalloy has a composition including 0.005 to 0.020 wt-% carbon, up to 1.00wt-% silicon, up to 2.00 wt-% manganese, from 9.0 to 13.0 wt-% nickel,17.0 to 19.0 wt-% chromium, 0.20 to 0.50 wt-% niobium, and 0.06 to 0.10wt-% nitrogen. The remainder of the composition of the low carbonstainless steel includes iron and may include one or more additionalcomponents. Table 1 below shows the composition of the new low-carbonstainless steel (LCSS) and other austenitic stainless steelstraditionally used to form hydrocarbon treating vessels.

TABLE 1 Composition of Low-Carbon Stainless Steel and Comparison withother Austenitic Stainless Steels for Hydrocarbon Conversion Vessels CSi P S Mn Ni Cr Nb N LCSS 0.005-0.020 1.00 0.045 0.030 2.00 9.0-12.017.0-19.0 0.20-0.50 0.06-0.10 Max Max Max Max 347H 0.04-0.10 1.00 0.0450.030 2.00 9.0-13.0 17.0-19.0 0.40-1.0 — Max Max Max Max 347 0.08 1.000.045 0.030 2.00 9.0-13.0 17.0-19.0 0.40-1.0 — Max Max Max Max Max

At least a portion of the interior surface of the hydrocarbon treatingvessel is formed of the low-carbon stainless steel alloy. The interiorsurface may include the walls of the vessel or may include surfaces ofother structures or apparatuses within the vessel. For example, wherethe vessel is a tube or pipe, the interior surface may include internalsurfaces, i.e. hydrocarbon contacting surfaces, of the walls of the tubeor pipe, and the surfaces of any flanges or welds that are exposedwithin the hydrocarbon treating vessel. Similarly, where the vesselincludes a reactor or other structure, the interior surface of thevessel may include interior surfaces of the walls of the vessel as wellas surfaces of reactor internal equipment and structures within thereactor that are subject to high temperatures and other materials withinthe reactor.

In one form, the process includes identifying one or more PTA-SCCaffected zones of an interior surface of the hydrocarbon treating vesselor vessels and employing these PTA-SCC affected zones formed from thelow carbon stainless steel alloy. The process may also includeidentifying another non-PTA-SCC affected portion of the interior surfaceof the hydrocarbon treating vessel and employing the non-PTA-SCC portionformed from another material, including for example traditional types304 or 347 stainless steel. In this manner, the low carbon stainlesssteel may be incorporated in areas where sensitization has beenidentified to result in PTA-SCC, and not in other areas that have notbeen identified as posing a significant risk of PTA-SCC, due to limitedexposure to heat to reduce the likelihood of sensitization or limitedexposure to moisture or oxygen or hydrogen sulfide, so that theformation of polythionic acid is not expected in these areas. In thismanner, fabrication costs may be reduced by reducing the amount of thespecialty low-carbon austenitic stainless used in forming the vessel. Aless expensive material may then be employed in other areas of thevessel where PTASCC is not identified as being problematic.

In another approach, the process may include forming all interiorsurfaces of the hydrocarbon treating vessel from the low-carbonaustenitic stainless steel. In yet another approach, interior surfacesof a plurality of hydrocarbon treating vessels within a hydrocarbontreating complex may be formed of the low carbon austenitic stainlesssteel. In the foregoing approaches, by incorporating the low-carbonstainless steel within the complex or a particular vessel, sensitizationmay be reduced. In this regard, the interior surfaces may resistcorrosion from polythionic acid and resulting PTA-SCC even if hydrogensulfide within the system is allowed to interact with oxygen ormoisture.

It has been found that welds within a complex vessel or connectinghydrocarbon treating vessels together can be particularly susceptible toPTA-SCC due to residual stresses that typically result during welding ofa material. Traditionally, post weld heat treatment has been required torelieve these residual stresses in the weld location in order to reducethe localized stresses that would otherwise promote PTA-SCC fromoccurring. It has been surprisingly found that by forming at least aninterior surface of the vessel with the low-carbon stainless steelalloy, the post weld heat treatment step may be avoided, thus savingtime and resources and reducing the cost of fabrication of the complex.

According to another approach, interior surfaces of the vessel areformed from the low-carbon stainless steel by employing an entirethickness of a vessel wall formed from the material. In this regard,sensitization is restricted or reduced through the entire thickness ofthe vessel wall. By another approach, the process includes employing aninterior surface of the hydrocarbon treating vessel that is formed fromthe low-carbon stainless steel and employing outer portions of thevessel walls formed from a second material. To this end, a low-carbonstainless steel coating may be applied on the interior surfaces of thevessel walls or internal equipment. In another example, a weld overlayor cladding formed from the low-carbon stainless steel alloy may be maybe employed on the interior surface of a second material to provide aninterior surface formed of the low-carbon stainless steel. In anotherexample, sheets formed from the low-carbon stainless steel may beattached to inner surfaces of an outer shell formed of a second materialby welding or other known methods for attaching the plates to the vesselwalls. Other methods for employing interior surface of the vessel formedfrom the low-carbon stainless steel while employing outer portions ofthe vessel wall formed from the second material are also contemplatedherein. Similar to the discussion above regarding forming PTA-SCCaffected zones from the low-carbon stainless steel, by forming theinterior surface of the vessel walls from the low-carbon stainless steelwhile forming outer portions of the walls from a second material,fabrication costs may be reduced by using a less expensive material. Inaddition, the second material may be selected to provide otherbeneficial characteristics such as superior strength or resistance toother environments or conditions that affect outer portions of thevessel.

Turning to more of the particulars, as the hydrocarbon stream flowsthrough the vessel, according to one approach, sulfur is present withinthe vessel, for example as a contaminant in the hydrocarbon stream or asH2S added to the hydrocarbon treating vessel to restrict coking withinthe vessel. During operation, the sulfur may form an iron sulfide scalelayer on the interior surface of the vessel. The formation of the ironsulfide scale on the interior surface of the vessel interacts withoxygen and moisture if the system is opened to the atmosphere withoutappropriate neutralization or purging, as has been previously done, toform polythionic acid. The polythionic acid is responsible for PTA-SCC.

In this regard, in one approach, the hydrocarbon treating complex orvessel is temporarily shut down, either during the predetermined timefor maintained operation of the hydrocarbon treating complex, or afterthe predetermined time. The process includes opening hydrocarbontreating complex or vessel to the atmosphere. The hydrocarbon treatingcomplex may be opened to the atmosphere, for example, to allow servicingor inspection. As mentioned above, in one approach, the hydrocarbontreating complex, or a vessel thereof is opened to the externalenvironment without neutralizing or purging the hydrocarbon treatingvessel so that the interior of the vessels are subjected to the externalenvironment. In this regard, oxygen and moisture are allowed to enterthe vessel and interact with the sulfur, hydrogen sulfide, and or ironsulfide scale within the vessel to form polythionic acid. In otherwords, during shutdown and startup procedures of the complexes, no stepsare taken to restrict the formation of polythionic acid within thevessel such that the polythionic is allowed to form.

It has been surprisingly discovered, however, that by forming interiorsurfaces of the vessel with the low-carbon stainless steel alloy, thepolythionic acid that is present within the vessel will not causepolythionic stress corrosion cracking for at least the predeterminedamount of time. It is believed that by employing the interior surface ofthe vessel formed of the low-carbon stainless steel alloy, the interiorsurface may actually be immune to sensitization and thus PTA-SCC. Not tobe bound by theory, it is believed that because sensitization of thestainless steel alloy does not occur in the high temperature operatingconditions present within the vessel, the occurrence of chromiumdepleted zones typically present as a result of sensitization areminimized such that the polythionic acid is not able to corrode thestainless steel alloy because the protective chromium layer remainsgenerally intact. In this manner, the process includes exposing thehydrocarbon treating complex or vessel to the external environmentwithout taking steps to reduce or restrict the formation of polythionicacid and controlling corrosion of an interior surface of the vessel bythe polythionic acid.

The amount of sensitization that occurs within the novel austeniticstainless steel alloy may be measure according to ASTM A262, Section 6Classification of Etch Structures and ASTM A262 Practice C CorrosionRate Nitric acid Test. The degree of sensitization may also bequantified per ASTM G108 Electrochemical Reactivation (EPR) test. Basedon the normalized charge (Pa) in units of coulombs/cm2, the degree ofsensitization can be determined (see an excerpt below for 304/304L). Inone approach, the Pa value of the novel austenitic stainless steel alloymay be below 0.4 indicating that only slight sensitization of theinterior surface portion will occur within the predetermined period oftime. In another approach, the Pa value of novel austenitic stainlesssteel alloy is below 0.10 indicating that no sensitization will occurafter the predetermined period of time. In another approach, the Pavalue may be below 0.05. In yet another approach, the Pa value may bebelow 0.01.

Table 2 below provides specific examples of hydrocarbon treatingprocesses in accordance with the present invention. The invention is notintended to be bound by these examples. Table 2 provides the source ofsulfur and the process temperature for each of the processes. In eachexample, the process is maintained for the predetermined amount of timeas described previously, and may be intermittently shut down or stoppedduring the predetermined amount of time. The table also indicateswhether hydrogen sulfide is present in the hydrocarbon stream orinjected into the process.

TABLE 2 Example Hydrocarbon Conversion Processes in Accordance with theInvention PROCESS REACTOR - Primary PROCESS FEED SULFUR PRODUCTS TEMP(C.) Processing Goal Formation of Natural Injected Renewable 230-455Deoxygenation (1^(st) renewable (triglyceride) co-feed Diesel stage) anddiesel from Oils/fats Isomerization (2^(nd) natural oils Stage)Formation of Natural Injected Renewable 230-455 Deoxygenation (1^(st)Renewable Jet (triglyceride) co-feed Jet stage) and Selective FuelOils/fats Cracking (2^(nd) Stage) Hydroprocessing - Naphtha, Present Lowsulfur 230-500 Sulfur removal Hydrotreating Kerosene, in feed Naphtha,Diesel, VGO, Kerosene, DAO, Resids, Jet Fuel, Coker Gas Diesel, or OilsVGO Hydroprocessing - Diesel, VGO, Present Low sulfur 230-500 Sulfurremoval and Hydrocracking DAO, Resids, in feed Naphtha, cracking (in oneor Coker Gas Kerosene, two stages) Oils Jet Fuel, Diesel, UCO Productionof Propane or Injected Propylene/ 550-700 Dehydrogenation olefins fromiso-Butane co-feed iso- paraffins Butylene Conversion of LPG InjectedBenzene/ 550-700 Dehydrocyclization LPG to Liquid (propane co-feedToluene Aromatics and butane) Olefin cracking C4-C6 Injected Ethylene/500-750 Cracking Olefins Co-feed Propylene High Naphthas InjectedAromatics/ 550-700 Dehydrogenation and temperature co-feed isoparaffinsisomerization to form reforming aromatics and branched paraffins

EXAMPLE

347AP stainless steel plate samples were prepared generally inaccordance with Example A3 in US Patent Publication No. 2010/0054983 andtested for sensitization and stress corrosion cracking. The 347APstainless steel sample prepared for testing included the compositionprovided in Table 1 below. The 347 AP material was cold rolled into aplate followed by heat treatment of 1080° C. The 347AP plate was thensectioned into smaller pieces, half of which were welded using acombination of tungsten inert gas (TIG) welding and shielded metal arcwelding (SMAW). The welded and un-welded pieces were then encapsulatedin a quartz vessel under vacuum to prevent oxidation during heattreatment. Four different sensitization treatments were then performedfor both welded and non-welded samples: a) 1 hour at 675° C., b) 10hours at 675° C., c) 2,000 hours at 565° C. and d) 10,000 hours at 565°C. For comparison purposes, non-welded plates formed of 347AP were alsotested at 565° C. for both 2000 and 10,000 hours and pieces of 304H wereincluded at 565° C. for 2000 hours.

TABLE 3 Composition of example low-carbon stainless steel alloy and thegeneral composition of the low-carbon stainless steel alloy showingranges of components. Weight Percent C Si Mn P S Ni Cr Nb N Sam- 0.0070.36 1.48 0.028 0.001 10.0 17.2 0.31 0.08 ple* Min 0.005 — — — — 9.017.0 0.20 0.06 Max 0.020 1.00 2.00 0.045 0.030 12.0 19.0 0.50 0.10

Sensitization testing was performed using ASTM A262 Practice A (oxalicacid grain boundary etching). The tests indicated that the 347AP sampleshad not sensitized after 10,000 hours at 565° C. and after 10 hours at675° C. In contrast, both tests showed that the 304H and 347 referencesamples had become sensitized after 2,000 hours at 565° C.

Susceptibility to intergranular corrosion was also assessed using ASTMA262 Practice C (boiling nitric acid). This test showed that in aboiling nitric acid solution, 347AP did experience intergranular attack,but, due to the lack of sensitization confirmed by the aforementionedtesting, it is believed this intergranular corrosion is due to thepreferential attack of precipitated phases other than Cr-carbides.

The time-temperature envelope where 347AP was found to not sensitize wasexpanded up to 565° C. to 10,000 hours and 675° C. to 10 hours.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention. Itshould be understood that the illustrated embodiments are exemplaryonly, and should not be taken as limiting the scope of the invention.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.Pressures are given at the vessel outlet and particularly at the vaporoutlet in vessels with multiple outlets.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

The invention claimed is:
 1. A process for treating a hydrocarbon streamcontaining hydrogen sulfide, the process comprising: flowing thehydrocarbon stream containing hydrogen sulfide through a hydrocarbontreating vessel; heating at least a portion of an interior surface ofthe vessel to a predetermined vessel temperature of 565° C. or greaterfor 1,000 hours or more; allowing a portion of the hydrogen sulfide tointeract with oxygen and air within the vessel to form polythionic acidand controlling stress corrosion cracking of the portion due to thepolythionic acid; controlling sensitization of the portion of thehydrocarbon treating vessel by employing a hydrocarbon treating vesselwith at least the portion thereof formed of a low-carbon stainless steelalloy having a Pa value below 0.10 comprising 0.005 to 0.020 wt-%carbon, 9.0 to 13.0 wt-% nickel, 17.0 to 19.0 wt-% chromium, 0.20 to0.50 wt-% niobium, and 0.06 to 0.10 wt-% nitrogen to restrictsensitization of the portion of the interior surface; and intermittentlystopping flowing the hydrocarbon stream, reducing the vessel temperaturebelow the predetermined temperature, and exposing the interior of thehydrocarbon treating vessel to an external environment including oxygenand moisture without neutralizing or purging the interior of thehydrocarbon treating vessel and without causing polythionic acid stresscorrosion cracking of the portion of the interior surface.
 2. Theprocess of claim 1, wherein the predetermined vessel temperature ismaintained within the hydrocarbon treating vessel between 565° C. and700° C.
 3. The process of claim 1, wherein the predetermined vesseltemperature is a range of temperatures between 565° C. and 700° C. 4.The process of claim 1, wherein the predetermined vessel temperature ismaintained within the hydrocarbon treating vessel for more than 5,000hours without sensitization of the portion of the interior surface ofthe vessel.
 5. The process of claim 1, further comprising identifyingzones of the interior surface of the hydrocarbon treating vessel thatare susceptible to sensitization and controlling sensitization of thezones by employing the hydrocarbon treating vessel having the zonesformed from the low-carbon stainless steel alloy and identifying otherzones of the interior surface of the hydrocarbon treating vessel thatare not susceptible to sensitization, and employing the hydrocarbontreating vessel having the other zones formed from another material. 6.The process of claim 1, further comprising employing a hydrocarbontreating vessel with an entire interior surface thereof formed from thelow-carbon stainless steel alloy.
 7. The process of claim 1, furthercomprising welding the interior surface of the vessel, wherein weldedmaterial is formed from a stainless steel alloy comprising 0.005 to0.020 wt-% carbon, 9.0 to 13.0 wt-% nickel, 17.0 to 19.0 wt-% chromium,0.20 to 0.50 wt-% niobium, and 0.06 to 0.10 wt-% nitrogen to controlsensitization of the welded material.